This invention relates to the art of heat treating moist pulverulent material such as raw material to be calcined for the preparation of cement and particularly deals with the thermal treatment of slurries of pulverulent raw cement material to minimize introduction of infiltrated air into the heat exchange system while maintaining high vacuum in the system.
It has heretofore been proposed to stop entry of infiltrated air into thermal heat treating devices for slurries and the like by introducing fine granular moist material into a gas conduit of a heat exchange system having two separators by means of a centrifugal roller. Such a device is described in Swiss Pat. No. 394,953 dated Dec. 15, 1965. According to this Swiss patent, the raw material is introduced into the system by a feed worm to a centrifugal roller and thin layers of the material are scraped from the roller and discharged into a stream of drying gas. However, a good distribution of the moist charging material in the stream of gas can only be obtained in this apparatus when the charging material was composed of a sprayable substance. Where the charging material is inclined to agglomerate, the agglomerated particles drop downwardly in the gas stream and interfere with efficient drying.
The present invention eliminates the above indicated disadvantages of the prior art by installing a blower in the gas conduit in advance of the point of materials supply. The blower is regulated relative to the conditions in the gas conduit so that at the point of materials supply, no differential pressure, or only a low differential pressure, prevails thereby preventing infiltration of outside air by simple construction devices. Further, the invention contemplates the use of a very long gas conduit leading to the last separator so that an efficient pre-drying of the material is obtained.
In order to maintain a uniform distribution of the charging material into the stream of gas, the gas conduit rises vertically from the point of introduction of the material up to the last separator. Further, the lower end of the vertical gas conduit receives a comminution device which commimutes coarse particles of the material which might drop downwardly in the gas stream to render them sufficiently fine so that they will be picked up by the gas stream and distributed in the separator.
The comminution device can be a centrifugal mill with a direction of rotation corresponding to the direction of gas flow for assisting the picking up of the comminuted material by the gas stream.
The centrifugal mill is preferably so constructed that the gases of the gas stream may flow therethrough to prevent caking of material in the mill.
There is also provided a combustion chamber receiving gases from a drying kiln which heats these gases and discharges them to the first and last separator and if desired also to a point between the blower and the charging point of the material to intensify the drying effect of the gases and making the system useful with very thin liquid slurries.
Another feature of the invention includes the provision of an air cooler receiving material from the kiln with the exhaust air being used for pre-drying the moist charging material and with particles of dust in this exhaust of the cooler being reintroduced into the system and precipitated thereby eliminating heretofore required cooler-exhaust air-dust-removal.
A further feature of the invention is the superimposing of the separators up to the last separator and providing a gas conduit leading to this last separator which has an elongated downturned leg and an elongated upturned leg with the material to be treated introduced in the lower area of the upturned leg thus providing a reliably operating and space-saving arrangement of separators and conduits.
To remove coarse constituents of the material to be treated from the gas stream before the last separator, the invention also contemplates the installation in the gas conduit between the material charging point and the last separator, a sifter or grader which discharges the coarse material back to the supply line.
Another feature of the invention is a provision of a branch conduit in the material discharge outlet of the last separator so that dry material separated out in this last separator may be mixed with moist charging material to effect a drying of the moist charging material.
Further, the supply of raw material to the system may take place either in the gas conduit leading to a centrifugal mill or to the centrifugal mill itself.
Other and further details, features and advantages of this invention will be apparent to those skilled in this art from the following detailed description of a preferred embodiment of the invention as shown in the attached drawing.